The main aim of this project is to study the regulation of energy expenditure and energy intake in humans and identify metabolic risk factors of weight gain in children and adults. To assess resting energy expenditure we use indirect calorimetry (metabolic carts and respiratory chamber). To assess energy expenditure in free-living condition we use doubly labeled water DWL). To assess energy intake and eating behavior we use food frequency questionnaires, vending machines, and taste tests. To determine the neuroanatomical correlates of hunger, taste and satiation we use positron emission tomography (PET). In the past year we have: a) expanded the study of the heritability of physical activity level (PAL) to 100 pairs of twins; b) completed a collaborative study to validate diet and PAL questionnaires in 35 Native American children using DLW (data currently being analyzed); c) continued a study to characterize eating behavior in pre-pubertal offspring of diabetic mothers; d) continued a study to characterize eating behavior in adult Pima siblings (ongoing); e) conducted a PET study in individuals with Prader-Willi syndrome (ongoing)